The present invention pertains to high pressure fuel injection pumps. More particularly, the invention is directed to improving fuel injection timing for high pressure fuel injection unit pumps or unit injectors.
Internal combustion engines may rely on high pressure fuel injection pumps to pressurize a supply of fuel for injection into the engine combustion chamber. The high pressure fuel injection pump designs available to accomplish fuel pressurization and injection vary widely. One known fuel injection pump design uses discrete fuel injection unit pumps each typically coupled to a single combustion chamber of the engine. Each unit pump includes a pumping chamber defined by a longitudinal pumping bore within the unit pump body and a pumping plunger disposed for reciprocation therein. The pumping chamber is terminated by a head assembly which is connected to the engine combustion chamber, typically by a high pressure line and fuel injector. A fuel supply port fluidly connects the pumping bore to a fuel supply source.
The pumping plunger has a pumping end and an opposing driven end. A cam follower assembly is disposed between the plunger pumping end and a rotatable cam. The rotatable cam acts against the cam follower assembly to periodically force the pumping plunger toward the head, thereby pressurizing the fuel within the pumping chamber for discharge to the engine combustion chamber. A spring biases the pumping plunger, and thereby the cam follower assembly, against the rotatable cam. The spring bias ensures that the pumping plunger and cam follower assembly maintain continuous contact with the cam, so that the pumping plunger periodically moves away from the head and thereby draws fuel from the supply port into the pumping chamber.
The cam is mechanically coupled in a well known manner to an engine crankshaft which is in turn mechanically coupled to engine pistons reciprocating within engine cylinders. In this manner, the rotational angle of the cam is in a fixed relationship to the linear position of the engine piston within its cylinder. Likewise, the rotational angle of the cam is mechanically related to the linear position of the pumping plunger within the pumping bore. The relationship of the cam with both the engine pistons and pumping plunger allows control of the timing of the plunger pumping stroke so that fuel can be injected into the engine combustion chamber when the engine piston is at a desired position in its linear travel. Typically, fuel is injected before the piston has reached the top of its stroke.
Control of fuel injection timing is important for engine cold starting and power output. Control of fuel supplied to the combustion chamber of an internal combustion engine by a fuel injection pump has also become increasingly important due to the demand for improved fuel economy and increasingly stringent legislation controlling emissions emanating from internal combustion engines. In particular, control of the timing at which the unit pump starts and ends the injection of fuel into the combustion chamber is important in meeting these demands. One known method for controlling the delivered fuel quantity in conjunction with the timing of the fuel injection event with a unit pump or unit injector provides the pumping plunger outside diameter with upper and lower helical channels. As the plunger reciprocates, the helical channel intermittently aligns with the supply port, or alternatively a spill port. As the pumping plunger travels toward the head the upper helical channel moves out of alignment with the fill port, generating high pressure in the pumping chamber, and the fuel injection event begins. As the pumping plunger continues movement toward the head, the lower helical channel is aligned with the fill port and the fuel injection event ends. Rotation of the pumping plunger within the pumping bore serves to adjust the timing for the alignment of the helical channels and fill/spill ports, thereby adjusting the delivered fuel quantity and timing of the fuel injection event.
It is an object of the invention to provide an additional mechanism for varying the timing of the fuel injection event.
It is another object of the present invention to provide a method and apparatus for controlling the timing of a fuel injection event, the method and apparatus providing an optimal combination of simplicity, reliability, efficiency and versatility.
It is yet another object of the invention to provide an apparatus for controlling the timing of a fuel injection event which contains the relationship between the linear position of the pumping piston and the rotational angle of the cam.
These and other objects and advantages of the present invention are achieved by the use of a fuel injector unit pump, driven by a cam that functions to supply fuel to an injector for an injection event. The fuel injector unit pump includes a body and a pumping plunger reciprocably disposed within the body and has a driven end. A cam follower assembly is provided for engaging the cam and includes an advance piston that engages the driven end of the pumping plunger for advancing or retarding the timing of the injection event. The advance piston is movable in response to fluid pressure controlled by an advance control. A follower return spring is disposed between the body and the cam follower assembly and a plunger return spring is nested with the follower return spring and between the body and the advance piston.
The advance piston is hydraulically actuated and is disposed between the rotatable cam and pumping plunger. In a retracted position the pumping plunger is separated from the cam rotational axis by a first distance. The first distance defines a relationship between the pumping plunger linear position, cam rotational angle and engine piston position. By pressurizing the advance piston, the advance piston is moved outwardly toward an extended position, which in turn displaces the pumping plunger away from the cam rotational axis. Since the position of the pumping plunger within the pumping bore determines fuel injection event timing, for the same cam rotational angle the fuel injection event timing will be different depending on whether the advance piston is retracted or extended. Naturally, the fuel injection timing is continuously variable within the range of advance piston displacement. The range of advance piston displacement is also known as advance authority. An advance piston displacement range of 3 mm is possible.
To avoid separation of the pumping plunger and cam follower assembly from the cam, a follower return spring with a high spring force and spring rate is often used. Given the relatively small advance piston size it is difficult to apply a sufficient hydraulic pressure against the advance piston to overcome the force of the follower return spring. A balance spring can be placed below the advance piston to nearly balance the force of the return spring; however, the high spring rates of the return and balance springs severely limit the advance authority achievable with this configuration. An increased advance authority is achievable by using a pair of nested return springs.
In accordance with another feature of the invention, an outer cam follower assembly return spring provides a high force through a follower spring seat against the cam follower assembly, thereby maintaining the cam follower assembly against the cam as the cam rotates. An inner plunger return spring with a low force and low spring rate acts through a plunger spring seat against only the advanced piston to prevent separation of the plunger from the advance piston. Since the advance piston is biased only by the plunger return spring, pressurized lubricating oil from the engine lubrication system can be routed through a hydraulic advance circuit to hydraulically actuate the advance piston.
A control device fluidly upstream or downstream of the advance piston controls pressure within the hydraulic advance circuit, thereby controlling actuation of the advance piston, and ultimately timing of the fuel injection event.
Preferably, the advance piston includes an annular channel or step at the piston crown. This step cooperates with an annular shoulder formed on the inside diameter of the follower spring seat to limit the maximum displacement of the advance piston, and thereby the ultimate advance authority achievable. Further, preferably, the follower spring seat incorporates a retainer such as tabs or a lip to retain the follower spring during assembly.